Nematic liquid crystal composition and bistable nematic liquid crystal display

ABSTRACT

A nematic liquid crystal composition of the present invention includes at least 20% by weight of one, or two or more compounds selected from compound group A; and 5 to 50% by weight of one, or two or more compounds selected from compound group B, wherein the relative proportions of the above-mentioned compounds used in the nematic liquid crystal composition is determined to simultaneously obtain the specific physical characteristics for the composition, and an anchoring breaking voltage U λ/4  is less than or equal to 25 V when the product (Δn·d) of the refractive index anisotropy (Δn) at 20° C. and the thickness (d) is 140 nm and the liquid crystal composition is confined between two substrates for a bistable nematic liquid crystal display in which at least one of the substrates have a weak zenithal anchoring force.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending application: “NEMATIC LIQUID CRYSTAL COMPOSITION AND BISTABLE NEMATIC LIQUD CRYSTAL DISPLAY” filed even date herewith in the names of NAKATA, Hidetoshi; KURIYAMA, Takeshi; TAKEUCHI, Kiyofumi; TAKATSU, Haruyoshi; STOENESCU, Daniel; DUBOIS, Jean-Claude; and DOZOV, Ivan as a national phase entry of PCT/JP2008/059493; which application is assigned to the assignee of the present application and incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a nematic liquid crystal composition that can be suitably applied to a bistable nematic liquid crystal display and to a bistable nematic liquid crystal display that uses the composition.

BACKGROUND ART

Liquid crystal displays (LCDs) are increasingly used in various display applications because they are thin, lightweight, and have low power consumption. Such LCDs have a structure in which a liquid crystal composition is confined between two substrates, at least one of which is transparent. The substrates are provided with an electrode layer on their internal faces (mutually facing surfaces) to impress an electric field on the liquid crystal composition and an alignment film in which an orientation treatment is performed that orients the liquid crystal. This orientation treatment serves to both arrange and anchor the liquid crystal composition on the alignment film.

By applying an electric field exceeding the Freedericksz transition voltage between the electrodes of the two substrates, the orientation of the liquid crystal composition varies under the action of the electric field. Due to birefringence of the liquid crystal composition, these orientation variations change the optical properties of the display, and moreover by using a polarizing plate it functions as a display device.

Such LCDs, called “classical” LCDs, have the following characteristics:

(1) When the external electric field is switched off after displaying information, the displayed information also disappears;

(2) the alignment film functions to align the liquid crystal molecules to be parallel with respect to the alignment film and has an extremely strong anchoring force with respect to the liquid crystal molecules. For that reason, even when an electric field is applied, the liquid crystal molecules in the vicinity of the alignment film basically maintain their parallel orientation with respect to the alignment film without reorienting to the direction of the electric field; and

(3) when the field is switched off, the state prior to the electric field application is reverted to.

A large amount of work has been done to improve the performances of the liquid crystal composition of classical LCDs by optimizing the physical properties including the temperature range, viscosity, elasticity, birefringence, dielectric anisotropy, Freedericksz transition voltage, etc. It is nearly impossible to optimize the above-mentioned characteristics with a single compound, and so mixtures combining a plurality of compounds are required (see Handbook of Liquid Crystals, Wiley-VCH Weinheim (1998)).

Furthermore, for these classical LCDs, the anchoring force with respect to the liquid crystal molecules of the alignment layer need not be strictly defined, with all that is required is that this anchoring be “strong”, in other words, greater than a given limiting value (L_(z)<15 nm, with L_(z) being defined below). Research has been conducted on this problem in obtaining strong anchoring in an alignment film, with several known alignment layer materials being used to provide strong anchoring (see Liquid Crystals—Applications and Uses, World Scientific Publishing Co. Pte. Ltd. Singapore (1990)).

In recent years research has been conducted on a new generation of nematic displays called “bistable” nematic displays. In these displays, the liquid crystal molecules have two stable textures without any applied voltage. The voltage is only applied for the time necessary to switch between these two orientation states. There is thus no need to keep applying a voltage in order to maintain the display. Due to its operating principle, this type of display consumes an amount of energy proportional to the number of image changes. Thus as the frequency of image changes drops, the power necessary for operation of the display tends towards zero. This type of display would therefore be extremely effective for mobile devices in which low power consumption is required.

Two kind of bistable displays have been proposed where the stable states are stabilized by the orienting film on the plates. One kind uses orienting films which orient the molecules in two directions, i.e. bistable orienting films, the other kind uses more simple orienting films which orient in one direction only, i.e. monostable orienting films. The switching between the two stable states of these displays is obtained by breaking the anchoring of the molecules at least on one orienting film: an applied field put the molecules on the surface in a direction where the surface torque is zero and the energy maxima. After removing the field the molecules close to the film return in a stable orientation driving the molecules in the bulk to one or the other stable states.

The display device developed by the ZBD Displays Ltd. (G. P. Bryan-Brown et al., Nature, 399,338 (1999)) uses a bistable orienting film: close to the film, in one stable state, the molecules are oriented nearly parallel to the plate; in the other stable state, the molecules are nearly perpendicular to the plate. The Orsay Solid State Physic Laboratory proposed two bistable nematic displays using bistable orienting surfaces which orient the molecules in two tilted states: French Patent Application, Publication No. 2663770 where the commutation uses a flexoelectric effect and French Patent Application, Publication No. 2657699 which uses an electrochiral effect.

Two bistable nematic displays using monostable orienting films and commuting with anchoring breaking have been developed: the Bistable Nematic (BiNem®) display by Nemoptic Ltd. in France (French Patent Application, Publication Nos. 2740893 and 2740894, and U.S. Pat. No. 6,327,017) and the SBiND display developed by LICET Ltd. in Italy (European Patent Application. Publication No. 0 773 468, U.S. Pat. No. 5,995,173 and Japanese Unexamined Patent Application, Publication No. H09-274205).

The switching principle of the BiNem® bistable display by Nemoptic Ltd. is diagrammatically shown in FIG. 1. It uses two textures, one uniform or slightly twisted texture T₀ in which the molecules are approximately parallel to each other (±20°), and the other texture T₁₈₀ that differs from the first by a twist of 180°±20°.

The nematic is chiralised with a spontaneous pitch p₀, chosen to be close to four times the thickness of the cell to equalize the energies of the two textures. In the absence of a voltage, these two states become minimum values in terms of energy. In the presence of a high voltage, anchoring of the molecules is broken on at least one of the plates (specifically, on the alignment film) and a nearly homeotropic orientation (H) of the liquid crystal molecules is obtained. This orientation state is a transition state (H), and can be switched to either of the two stable states (T₀, T₁₈₀). Slowly cutting the voltage can result in change to state T₀ by elastic coupling between molecules close to the two surfaces, and quickly cutting the voltage will lead to state T₁₈₀ by hydrodynamic coupling.

Bistable displays commuting by anchoring breaking require special properties of the liquid crystal mixture and the orienting film:

1) The anchoring on at least one alignment film has to be weak to allow the breaking by an applied field compatible with the driving electronics and the electrochemical properties of the different compounds of the nematic mixture.

2) The anchoring on the film can not be too weak because the liquid crystal textures in the stable states are stabilized by the anchoring. To maintain the textures, the anchoring torque need to be higher than the elastic torque applied on the surface by the bulk textures in the stable states.

3) The electrochemical stability of the different compounds of the mixture has to be higher to that for classical LCD displays. Indeed in classical LCD the applied voltage distorts only the bulk texture; it is close to two or three times the Freedericksz transition voltage, the minimum voltage to distort the texture maintained by the nematic elasticity. The breaking of the anchoring, taking into account the condition of the texture stability, needs a voltage almost ten times of the Freedericksz transition voltage.

4) The viscosity and the elastic constants of the mixture determine the optical response time of the displays. In the case of selection of the state by hydrodynamic coupling, these two parameters are fundamental also for the commutation.

5) A high optical refractive index anisotropy (0.14 to 0.20) has to be obtained to achieve the good contrast with a cell thickness smaller than in the classical LCD display. On the other hand, in the case of a reflection mode that uses one polarizing plate, since the optical distance (the value corresponding to the cell thickness) is doubled, in order to keep the product of Δn and d constant, a liquid crystal composition is required with a small value of Δn in the range of 0.06 to 0.14.

For a bistable display using anchoring breaking with a given mixture and a given anchoring, the voltage to break the anchoring is proportional to the thickness: to lower the voltage a small thickness is compulsory.

6) The nematic temperature range has to be wider than the targeted operating range. Indeed often the whole set of properties listed just before are not satisfied in the whole nematic range of the liquid crystal mixture: the range (ΔT_(N)) limited by T_(N-I) (the nematic-isotropic transition temperature) and T_(X-N) (the transition temperature towards the nematic phase from more ordered liquid crystal phases or vitreous or crystalline solid phases). To obtain all these properties, in a technically acceptable temperature range (50° to 80° centred on the ambient temperature), the mixtures need to have a nematic temperature range ΔT_(N) wider than this operating temperature range.

The anchoring and anchoring breaking concepts for liquid crystal molecules on surfaces are highly technical, and they can be defined. The orientation of the liquid crystal molecules by surfaces is called anchoring. The source of anchoring is anisotropy of the interaction between the liquid crystal compound and the surface. Anchoring can be characterized by the directionality induced by the surface to which the liquid crystal molecules are adjacent and the strength thereof. This direction is called the easy axis, and the direction n₀ of the easy axis is defined by the azimuth angle (Φ₀) and the zenithal angle (θ₀) (see FIG. 2). The average orientation direction of nematic liquid crystal molecules is drawn towards the easy axis. If there is no external influence, the liquid crystal molecules are oriented parallel to the easy axis to minimize the interaction energy with the surface. This energy (anchoring energy) may be written as follows as a first approximation (A. Rapini and M. Papoular, J. Phys. (Fr) C4, 30, 54-56 (1969)):

$\begin{matrix} {{g\left( {\theta,\phi} \right)} = {{\frac{W_{z}}{2}{\sin^{2}\left( {\theta - \theta_{0}} \right)}} + {\frac{W_{A}}{2}{\sin^{2}\left( {\phi - \phi_{0}} \right)}}}} & (1) \end{matrix}$

where θ and Φ are the zenithal angle and the azimuth angle, respectively, of the nematic director on the surface, and W_(Z) and W_(A) are the surface densities of the zenithal and azimuth anchoring energies, respectively.

The azimuthal anchoring energy W_(A) depends more on the anisotropy induced on the surface by treatments, than on the nature of the nematic materials. Even if it is compulsory in a bistable liquid crystal display to achieve a sufficient azimuthal anchoring to maintain twisted textures, we will not develop this subject.

The bistable displays commuting by anchoring breaking use more often the zenithal anchoring breaking. We will focus on this phenomenon.

The zenithal anchoring energy W_(Z) depends strongly on the chemical properties of the surface and of the nematic material. On most solid surfaces, the zenithal anchoring energy is one or two orders of magnitude higher than the azimuthal anchoring energy.

If the orientation of the director in the volume is different from the direction of the easy axis, the surface energy is no longer zero and the result is also energy dependent on an elasticity factor. The surface energy can be characterized by its extrapolation length that is the ratio between the bulk elasticity factor and the anchoring energy. The extrapolation length of the zenithal anchoring is denoted by L_(Z)=K₃₃/W_(Z) where K₃₃ is the bend elastic constant of the liquid crystal. In practice, a zenithal anchoring is considered as being strong if L_(Z)<15 nm and weak if L_(Z)>25 nm.

The orientation of liquid crystal molecules may be modified by external electrical or magnetic fields. For example, by applying an electric field that is perpendicular to the surface of the substrate, when liquid crystal molecules in the cell have positive dielectric anisotropy, they are oriented along the field (θ=0), and in the absence of an electric field, they orient to be nearly parallel to the surface of the substrate (θ=approximately 90°). On the surface, the director zenithal angle changes continuously as a function of the field, and θ becomes zero if the field exceeds the critical field E_(C). This state is called zenithal anchoring break, since the director of the liquid crystal molecules close to the surface is no longer affected by an anchoring torque or an electric torque. The critical field is shown as the equation formula (2).

$\begin{matrix} {E_{c} = \frac{W_{z}}{\sqrt{K_{33}{\Delta ɛ}}}} & (2) \end{matrix}$

In the formula (2), W_(Z) is the zenithal anchoring energy, K₃₃ is the bending elastic constant, and Δ∈ the dielectric anisotropy (relative to the dielectric constant ∈₀ of a vacuum).

This critical field strength E_(c) is the field is necessary to drive devices that utilize zenithal anchoring breaking. Mixtures with a high value of Δ∈ and high bending elasticity but a weak zenithal anchoring energy are necessary to control such devices.

In practice, the useful magnitude, in the case of zenithal anchoring breaking displays, is the voltage U_(z) that causes zenithal anchoring breaking. That is, the product of the critical field and the thickness of the cell. Usually, the thickness of bistable nematic displays cells is adjusted so that their birefringence is equal to the one-fourth wavelength of light at the center of their passband. To characterise the zenithal anchoring, the breaking voltage threshold U_(λ/4) is used. It is the breaking voltage U_(z) of a cell of optical thickness λ/4. U_(λ/4) is shown as the mathematical formula (3).

$\begin{matrix} {U_{\lambda/4} = {\frac{\lambda\; W_{z}}{4\Delta\; n\sqrt{K_{33}{\Delta ɛ}}} = {\frac{\lambda}{4\Delta\;{nL}_{z}}\sqrt{\frac{K_{33}}{\Delta ɛ}}}}} & (3) \end{matrix}$

In the formula (3), λ is the wavelength of light at the center of the passband, W_(z) is the zenithal anchoring energy, L_(z) is the zenithal anchoring extrapolation length, Δn is the refraction anisotropy in the wavelength λ, K₃₃ is the bending elastic constant, and Δ∈ is the dielectric anisotropy. The inventors consider that zenithal anchoring is weak when the breaking voltage U_(λ/4) is a voltage that can be supplied, within the temperature range, by a driver that is currently used ordinarily. In practice, this can be represented by the empirical rule stating that anchoring is weak if U_(λ/4) is less than or equal to 25 volts.

The zenithal anchoring energy depends on the material of the alignment film, the method of the surface treatment, the liquid crystal composition used, and the temperature.

The nature of the alignment film can greatly influence the zenithal anchoring energy. The polyimide orientation films used in classical LCDs show strong zenithal anchoring energy for most of the different families of nematic compounds. For example, on a commercially available polyimide orientation film (SE140 made by Nissan Chemicals Co.), L_(Z)=7 nm for the nematic compound pentyl-cyanobiphenyl (5CB), zenithal anchoring is strong.

On the other hand, Nemoptic Ltd. has developed copolymer films that provides weak zenithal anchoring for 5CB (L_(Z)>25 nm at 20° C.) and for other nematic compounds (European Patent Application, Publication No. 1 259854 and U.S. Pat. No. 7,067,180). In both patent documents, by standard method such as rubbing, a medium or strong azimuthal anchoring is simultaneously obtained leading to a good stability for both T₀ and T₁₈₀ textures.

Japanese Unexamined Patent Application, Publication No. 2005-133057 discloses an example of a liquid crystal composition in which the zenithal anchoring is weak. By combining nemoptic copolymer films with specific liquid crystal compositions, a low U_(λ/2) is obtained, and combinations having a nematic phase over a wide temperature range are also disclosed. In fact optimized anchoring properties of bistable displays depend on both liquid crystal alignment layer and liquid crystal mixture, but some liquid crystal mixtures can lead to good anchoring properties compatible with different types of alignment layers. However, in a bistable nematic display that uses zenithal anchoring breaking, the temperature range of the nematic phase and the operating temperature range of the display are not in a proportional relation. That is, at room temperature, even when the operating voltage is low, if the temperature dependency is high, the operating temperature range is in effect narrowed. In reality, a bistable nematic display that can operate over a wide temperature range has not been achieved. There is therefore a need to find a combination of liquid crystal compositions that can be activated over a wide operating temperature range.

Also, in the case of a reflective bistable nematic liquid crystal display that uses one polarizing plate, a small value of the refractive index anisotropy Δn in the range of 0.06 to 0.14 is required, as described above. The refractive index anisotropy Δn in Japanese Unexamined Patent Application 2005-133057 described above is greater than 0.14, and so is not suitable for a reflective bistable nematic liquid crystal display. A liquid crystal composition that has a low refractive index anisotropy Δn in the range of 0.06 to 0.14, so that low-voltage driving is possible in a bistable nematic liquid crystal display.

The present invention was achieved in view of the aforementioned circumstances, and has as its object to provide a nematic liquid crystal composition that has a wide operating temperature range when used for a bistable nematic liquid crystal display and a bistable nematic liquid crystal display that uses the composition.

DISCLOSURE OF THE INVENTION

The present invention obtained the following findings as a result of studying various combinations of liquid crystal compounds to solve the aforementioned problem.

A nematic liquid crystal composition comprising:

-   a) at least 20% by weight of one or more compounds selected from the     compound group A:

wherein:

R¹ is an alkyl group or alkenyl group having 2 to 7 carbon atoms, in which, in addition, one or more methylene groups existing in the alkyl group or alkenyl group may be substituted each independently with an —O—, —COO— or —OCO— where O atoms are not directly linked to each other,

Z¹ represents —CH₂CH₂—, —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, or a single bond,

Z² represents —CH₂CH₂—, —CH═CH—, —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, or a single bond,

Z³ represents —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, —C≡C—, or a single bond,

Z⁴ and Z⁵ are each, independently of one another, —CH₂CH₂—, —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, or a single bond,

X¹ is a cyano group, a fluorine atom, a chlorine atom, a trifluoro methyl group, a trifluoro methoxy group, or a difluoromethoxy group (—OCHF₂),

Y¹, Y², Y³, and Y⁴ are each, independently of one another, a hydrogen atom, a fluorine atom, a chlorine atom, a trifluoro methyl group, or a trifluoro methoxy group,

ring T¹ is a 1,4-phenylene group or a 1,4-cyclohexylene group, in which among these groups the 1,4-phenylene group may be unsubstituted or may have one or more of a fluorine atom, a chlorine atom, a methyl group, a trifluoro methyl group, or a trifluoro methoxy group as a substituent group,

m is 0 or 1, and

p and q are 0 or 1, provided that p+q is 0 or 1;

-   b) 5 to 50% by weight of one or more compounds selected from the     compound group B:

wherein:

R² and R³ are each, independently of one another, an alkyl group having 1 to 7 carbon atoms or an alkenyl group having 2 to 7 carbon atoms, in which, in addition, one or more methylene groups existing in the alkyl group or alkenyl group may be substituted each independently with an —O—, —COO— or —OCO— where O atoms are not directly linked to each other,

Z⁶ is —CH₂CH₂—, —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, or a single bond,

Z⁷ is —CH₂CH₂—, —CH═CH—, —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, or a single bond,

Z⁸ is —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, —C≡C— or a single bond,

Z⁹ and Z¹⁰ are each, independently of one another, —CH₂CH₂—, —COO—, —OCO—, or a single bond

Y⁷ to Y¹⁴ are each, independently of one another, a hydrogen atom, a methyl group, a fluorine atom, or a chlorine atom, and

rings T² and T³ are each, independently of one another, a 1,4-phenylene group or a 1,4-cyclohexylene group, in which among these groups the 1,4-phenylene group may be unsubstituted or may have one or more of a fluorine atom, a chlorine atom, a methyl group, a trifluoro methyl group, or a trifluoro methoxy group as a substituent group, wherein

the relative proportions of the above-mentioned compounds used in the nematic liquid crystal composition is determined to simultaneously obtain the following physical characteristics for the composition:

-   -   a nematic-isotropic liquid transition temperature (T_(N−I)) of         the liquid crystal composition of greater than or equal to 50°         C.,     -   a nematic temperature range (ΔT_(N)) of the liquid crystal         composition of greater than or equal to 50° C.,     -   a dielectric anisotropy of larger than or equal to 8×10⁻¹¹ F/m         at 20° C., and     -   a weak zenithal anchoring force on at least one of two         substrates for a bistable nematic liquid crystal display that         confine the composition, being defined by an anchoring breaking         voltage U_(λ/4) of less than or equal to 25 volts for a cell         with a thickness (d) such that the product (Δn·d) of the         refractive index anisotropy (Δn) and the thickness (d) is 140 nm         at 20° C.

The liquid crystal composition of the present invention can be used for a bistable nematic liquid crystal display in which it is confined between two substrates of which at least one of the substrates has a weak zenithal anchoring and can obtain a bistable nematic liquid crystal display that has a wide operating temperature range.

Also, it is possible to obtain a liquid crystal composition that can be driven at a low voltage in a reflective bistable nematic liquid crystal display that uses one polarizing plate, with the liquid crystal composition being confined between two substrates, of which one has a weak zenithal anchoring and a reflective layer is provided on the other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the operating of the bistable nematic liquid crystal display by anchoring breaking.

FIG. 2 is a drawing that explains the direction of the anchoring.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

1 glass substrate, 2 ITO electrode layer, 3 alignment film with strong anchoring, 4 alignment film with weak anchoring

BEST MODE FOR CARRYING OUT THE INVENTION

One example of the present invention shall be described below.

In the liquid crystal composition of the present invention, compounds of general formulas AI to AV that belong to the aforementioned compound group A are compounds that have a polar group on the terminal end, and are important compounds for causing anchoring breakage. Also, they are liquid crystal compounds with a small refractive index anisotropy (Δn) and so are compounds that are useful for obtaining a liquid crystal composition with a low refractive index anisotropy (Δn). Although the composition must contain at least one of the compounds represented by general formulas AI to AV, it is more effective in terms of lowering the driving voltage and widening the operating temperature range if it contains two or more of those compounds.

A particularly preferred case of the compound represented by general formulas AI to AV is given below.

X¹ is preferably a cyano group a fluorine atom, or a trifluoro methoxy group.

Z¹ is preferably —COO—, —OCO—, —CF₂O—, —OCF₂—, or a single bond.

Z² is preferably —CH₂CH₂—, —CH═CH—, or a single bond.

Z³ is preferably —COO—, —OCO—, —CF₂O—, —OCF₂—, or a single bond.

Z⁴ and Z⁵ are preferably single bonds.

Y¹, Y², Y³, and Y⁴ are each, independently of one another, preferably a hydrogen atom or a fluorine atom.

Ring T¹ is preferably a 1,4-cyclohexylene group.

R¹ is preferably an alkyl group or alkenyl group having 2 to 6 carbon atoms, in which, in addition, one, or two or more methylene groups existing in the alkyl group or alkenyl group may be substituted each independently with an —O—, —COO— or —OCO— where O atoms are not directly linked to each other, and more preferably an alkyl group or alkenyl group having 2 to 5 carbon atoms.

Also, preferable cases of the general formulas AI to AV are given below.

In general formula AI, preferably X¹ represents a cyano group, Z¹ represents a single bond, and Y¹ and Y², independently of one another, represent a hydrogen atom or a fluorine atom.

In general formula AII, preferably X¹ represents a cyano group, a trifluoro methoxy group (—OCF₃), or a fluorine atom, Z¹ represents —COO—, —OCO—, —CH₂CH₂—, —CF₂O—, —OCF₂—, or a single bond, and Y¹ and Y², independently of one another, represent a hydrogen atom or a fluorine atom.

In general formula AIII, preferably X¹ represents a cyano group or a fluorine atom, Z² represents —CH₂CH₂—, —CH═CH—, or a single bond, and Y¹ and Y², independently of one another, represent a hydrogen atom or a fluorine atom.

In general formula AIV, preferably X¹ represents a cyano group or a fluorine atom, Z¹ represents a single bond, Z³ represents —COO—, —OCO—, —CF₂O—, —OCF₂—, or a single bond, and Y¹ and Y², independently of one another, represent a hydrogen atom or a fluorine atom.

In general formula AV, preferably p is 0, X¹ represents a cyano group or a fluorine atom, Z² and Z⁴ represent single bonds, and Y¹ and Y², independently of one another, represent a hydrogen atom or a fluorine atom.

Also, in general formula AI, a compound in which X¹ is a cyano group, Z¹ is a single bond, and Y¹ and Y² are fluorine atoms has a low refractive index anisotropy (Δn), and since this is particularly preferable in terms of a low driving voltage and widening the operating temperature range, the liquid crystal composition of the present invention preferably contains at least 5% by weight of this compound.

More specifically, the following compounds are preferred.

In formulas C-1 to C-96, R⁴ is an alkyl group or alkenyl group having 2 to 6 carbon atoms, in which, in addition, one, or two or more methylene groups existing in the alkyl group or alkenyl group may be substituted each independently with an —O— where O atoms are not directly linked to each other. In addition, the alkyl group or alkenyl group is preferably a straight chain alkyl group or alkenyl group.

In the formulas C-1 to C-96, the compounds represented by (C-3), (C-38), (C-39), (C-41), and (C-42) have a low refractive index anisotropy (Δn), and are compounds which are particularly preferable in terms of a low driving voltage and widening the operating temperature range. Among others, the compound represented by (C-3) is more preferably, and containing at least 5% by weight of this compound is still more preferable.

The liquid crystal composition of the present invention must contain at least 20% by weight of one, or two or more compounds selected from compound group A represented by general formulas AI to AV.

Also, the liquid crystal composition preferably contains at least 25% by weight of one, or two or more compounds selected from compound group A represented by general formulas AI to AV.

Among the compound group A represented by general formulas AI to AV, the content of compounds with a dielectric anisotropy larger than or equal to 3.5×10⁻¹⁰ F/m is preferably not more than 25% by weight.

In these examples, in the case of mixing the compound group A or two or more compounds corresponding to general formulas thereof, the aforementioned values refer to the total content of the two or more compounds. Also, in the case of containing two or more of the compounds represented by general formulas AI to AV, R¹, X¹, Y¹ to Y⁶, Z¹ to Z⁵, T¹, m, p, and q of each of the compounds may be the same or different from each other.

In the liquid crystal composition of the present invention, compounds of general formulas BI to BX that belong to the aforementioned compound group B are compounds that do not have a polar group on the terminal end.

A particularly preferred case of the compound represented by general formulas BI to BX is given below.

R² and R³ are preferably an alkyl group having 1 to 6 carbon atoms or an alkenyl group having 2 to 6 carbon atoms, in which, in addition, one, or two or more methylene groups existing in the alkyl group or alkenyl group may be substituted each independently with an —O—, —COO— or —OCO— where O atoms are not directly linked to each other, and more preferably an alkyl group having 1 to 6 carbon atoms or an alkenyl group having 2 to 6 carbon atoms.

Z⁶ is preferably —COO—, —OCO—, —CH₂CH₂—, or a single bond, and more preferably —COO—, —OCO—, or a single bond.

Z⁷ is preferably —COO—, —OCO—, —CH₂CH₂—, or a single bond.

Z⁸ is preferably —COO—, —OCO—, —C≡C—, or a single bond.

Z⁹ and Z¹⁰ are preferably —COO—, —OCO—, —CH₂CH₂—, or a single bond.

Rings T² and T³ are preferably a 1,4-cyclohexylene group.

Y⁷ to Y¹⁴ are preferably a hydrogen atom or a fluorine atom.

More specifically, the following compounds are preferred.

In formulas II-1 to II-26, R⁵ and R⁶ represent an alkyl group having 1 to 6 carbon atoms or an alkenyl group having 2 to 6 carbon atoms, in which, in addition, one, or two or more methylene groups existing in the alkyl group or alkenyl group may be substituted each independently with an —O— where O atoms are not directly linked to each other, and Y³¹ to Y³⁵ represent a hydrogen atom, a fluorine atom, or a methyl group.

The liquid crystal composition of the present invention must contain 5 to 50% by weight of one, or two or more compounds selected from compound group B represented by general formulas BI to BX.

Also, the liquid crystal composition more preferably contains 5 to 40% by weight of one, or two or more compounds selected from compound group B represented by general formulas BI to BX.

In these examples, in the case of mixing the compound group B or two or more compounds corresponding to general formulas thereof, the aforementioned values refer to the total content of the two or more compounds. Also, in the case of containing two or more of the compounds represented by general formulas BI to BX, R², R³, Y⁷ to Y¹⁴, Z⁶ to Z¹⁰, T², and T³ of each of the compounds may be the same or different from each other.

The nematic-isotropic liquid transition temperature (T_(N-I)) of the liquid crystal composition must be greater than or equal to 50° C. (T_(N-I)) of the liquid crystal composition is more preferably greater than or equal to 60° C., and still more preferably greater than or equal to 70° C.

Also, in order for the liquid crystal display to realize a wide actuation temperature range, the temperature range (ΔT_(N)) in which the liquid crystal composition exhibits a nematic phase must itself be wide. For this reason, the temperature range (ΔT_(N)), which is defined by the difference between the upper limit temperature (T_(N-I)) (the nematicliquid crystalisotropic liquid transition temperature) and the lower limit temperature (T_(X-N)) (the transition temperature towards the nematic phase from more ordered liquid crystal phases or vitreous or crystalline solid phases), is preferably greater than or equal to 50° C., more preferably greater than or equal to 70° C., and still more preferably greater than or equal to 80° C.

Also, the lower limit temperature (T_(X-N)) at which the liquid crystal composition exhibits a nematic phase is preferably less than or equal to 0° C., more preferably less than or equal to −10° C., and still more preferably less than or equal to −20° C.

Since the greater the dielectric anisotropy (∈₀Δ∈) the lower the actuation voltage, the dielectric anisotropy of the liquid crystal composition at 20° C. must be greater than or equal to 8×10⁻¹¹ F/m. However, when it becomes excessively large, problems with reliability occur. For this reason, the value of the dielectric anisotropy at 20° C. is preferably 8×10⁻¹¹ to 6×10⁻¹⁰ F/m, more preferably 1.3×10⁻¹⁰ to 4.5×10⁻¹⁰ F/m, and still more preferably 1.7×10⁻¹⁰ to 3.5×10⁻¹⁰ F/m. Moreover, ∈₀Δ∈ is more preferably 1.3×10⁻¹⁰ to 3.0×10⁻¹⁰ F/m (Δ∈ of 15 to 35). ∈₀ is the dielectric constant of vacuum.

The refractive index anisotropy (Δn) is an important parameter for obtaining contrast. In the case of using a reflective bistable nematic liquid crystal display that uses a single polarizing plate, Δn at 20° C. is preferably 0.06 to 0.14, more preferably 0.06 to 0.13, and still more preferably 0.07 to 0.13. Also, it is more preferable that simultaneously with Δn being in the range of 0.06 to 0.13 at 20° C., ∈₀Δ∈ is in the range of 1.3×10⁻¹⁰ to 3.0×10⁻¹⁰ F/m.

Also, the anchoring breaking voltage U_(λ/4) (as represented by Eq. 3) must be less than or equal to 25 volts for a cell with thickness (d) such that the product of the refractive index anisotropy (Δn) and the thickness (d) is 140 nm (at a temperature of 20° C.) and the composition is confined between two substrates for a bistable nematic liquid crystal display in which at least one of the substrates has a weak zenithal anchoring. The anchoring breaking voltage U_(λ/4) is more preferably less than or equal to 20 V when the product of (An) and (d) is 140 nm (at a temperature of 20° C.).

In the liquid crystal composition of the present invention:

a) the liquid crystal composition is confined between two substrates of which at least one has a weak zenithal anchoring, the substrates have electrodes on their mutually facing inside surfaces so as to impress an electric field on the liquid crystal composition between the two substrates; and at least one substrate and electrode are optically transparent;

b) a process is applied to form at least two alternatively different textures that are stable or metastable under the absence of an electric field, with one of the textures being untwisted or twisted in a of range −90° to +90°, while the other texture is twisted at an angle close to 180°;

c) the product of (Δn) and (d) at 20° C. (wherein d represents the thickness of the liquid crystal composition layer, and Δn represents the refractive index anisotropy of the liquid crystal composition) is λ₀/4 (wherein λ₀ represents the center wavelength of the effective spectrum band of a display device).

d) by breaking the anchoring with at least one of these two substrates, it is possible to switch between the aforementioned different textures, and after the electric field is removed, it can be retained by either texture, and so is useful for a bistable nematic liquid crystal display device (for example, the BiNem® display shown in FIG. 1).

In the liquid crystal display of the structure shown in FIG. 1, substrate 1 is a glass substrate having an electrode layer 2 that consists of a transparent conductive material such as indium tin oxide (ITO). The anchoring by alignment films 3, 4 of the two substrates 1, 1 is preferably monostable for both. Also, at least one alignment film has weak zenithal anchoring. Letting the extrapolation length of the zenithal anchoring be L_(Z), zenithal anchoring is considered as being strong when L_(Z) is less than 15 nm and weak if L_(Z) greater than 25 nm.

Alignment film materials normally used as the alignment film 3 having strong zenithal anchoring include polyimide, polyamide, and SiO vapor-deposited film. A material consisting of a polyvinyl chloride (homopolymer) or a vinyl chloride copolymer (copolymer), as disclosed for example in French Patent Application FR 2817977, is suitable as the alignment film 4 that has weak zenithal anchoring. Monomers other than vinyl chloride used for a vinyl chloride copolymer include vinyl acetate, vinyl ether, acrylic ester, etc.

Since the present invention uses the aforedescribed liquid crystal composition as the liquid crystal composition confined between substrates of the liquid crystal display, it is possible to obtain a bistable nematic liquid crystal display with a wide operating temperature range.

The alignment film 4 with weak zenithal anchoring preferably has weak anchoring that is defined by an anchoring breaking voltage U_(λ/4) being less than or equal to 25 volts for a cell with a thickness d such that the product of the refractive index anisotropy (Δn) at 20° C. and the thickness (d) is 140 nm. Thereby, the liquid crystal composition can adopt at least two different stable states with the weak zenithal anchoring, and can achieve switching between the two different stable states when a suitable electric signal is applied on the electrode layers 2. This switching is preferably of the anchoring breaking type.

Also, the two different stable states are preferably a first stable texture (T₀) that is uniform or slightly twisted, in which the molecules of the liquid crystal composition are at least approximately parallel to each other, and a second stable texture (T₁₈₀) that differs from the first by a twist of 180° or an angle close to that. Thereby, when a suitable electric signal is applied to the two electrode layers 2, switching between (T₀) and (T₁₈₀) can be achieved. This switching is preferably achieved by breaking of the zenithal anchoring on at least one of the substrates.

Furthermore, in the liquid crystal composition of the present invention

e) the liquid crystal composition is confined between two substrates of which one has a weak zenithal anchoring and the other has a reflective layer, the substrates having electrodes on their mutually facing inside surfaces so as to impress an electric field on the liquid crystal composition between the two substrates; and at least the one substrate having the weak zenithal anchoring and the electrode thereof are optically transparent;

f) a process is applied to form at least two alternatively different textures that are stable or metastable under the absence of an electric field, with one of the textures being untwisted or twisted in a range of −90° to +90°, while the other texture is twisted at an angle close to 180′;

g) the product of (Δn) and (d) at 20° C. (wherein d represents the thickness of the liquid crystal composition layer, and Δn represents the refractive index anisotropy of the liquid crystal composition) is λ₀/4 (wherein λ₀ represents the center wavelength of the effective spectrum band of a display device).

h) by breaking the anchoring with at least one of these two substrates, it is possible to switch between the aforementioned different textures, and after the electric field is removed, it can be retained by either texture, and so is useful for a reflective bistable nematic liquid crystal display that uses a single polarizing plate.

For a reflective bistable nematic liquid crystal display that uses a single polarizing plate, a transparent substrate is used as the substrate 1 having the alignment film 4 with weak zenithal anchoring, while a reflective layer is provided on the other substrate in the liquid crystal display with the structure shown in FIG. 1. Then, one polarizing plate (not illustrated) is installed.

The present invention uses the liquid crystal composition of the present invention described above as the liquid crystal composition that is confined between the substrates of a reflective bistable nematic liquid crystal display that uses a single polarizing plate. Therefore, it is possible to obtain a bistable nematic liquid crystal display with a wide operating temperature range and a low driving voltage.

In the bistable nematic liquid crystal display of the present invention, the product of the refractive index anisotropy (Δn) and the thickness (d) of the cell is preferably 120 to 200 nm at 20° C., and more preferably 140 to 200 nm. Since the refractive index anisotropy (Δn) at 20° C. is determined by the liquid crystal composition, it is possible to attain the desired product of (Δn) and (d) by adjusting the gap between the substrates with a spacer.

EXAMPLES

The present invention shall be explained in greater detail below with examples, but the present invention is not limited by these examples. Also, “%” in the compositions of the examples and comparative examples below means “% by weight”. Also, unless otherwise specified, the nematic phase temperature range means the temperature range (temperature difference) from the solid phase or smectic phase-nematic phase transition temperature to the nematic phase-isotropic liquid phase transition temperature.

T_(N-I): nematic phase-isotropic liquid phase transition temperature

T_(X-N): solid phase or smectic phase-nematic phase transition temperature

Δ∈: dielectric anisotropy at 20° C.

Δn: refractive index anisotropy at 20° C.

Example 1

Two glass substrates with ITO were prepared as substrates to confine the liquid crystal composition, with SiO₂ (thickness: 107 nm) being vapor deposited on one of the glass substrates with ITO in order to obtain strong zenithal anchoring force of a high tilt, while a polymer (vinyl chloride isobutyl-vinyl-ether copolymer) represented by formula (G)

was coated with the method described below to a thickness of 20 nm on the other substrate in order to obtain a weak zenithal anchoring.

Step 1: A solution of the polymer diluted to 0.75% by weight in a 50/50 mixture of N-methylpyrolidone and butoxy ethanol is applied by spin coating.

Step 2: Annealing is carried out at 150° C. for 1.5 hours.

Step 3: Ultraviolet irradiation (λ=254 nm) is carried out for 2 hours using a 100 W mercury-vapor lamp.

Step 4: Annealing is performed at 150° C. for 30 min.

Step 5: A brushing process (rubbing) with a roller covered with a cloth velvet is carried out to induce azimuthal anchoring.

The two substrates are bonded together with a 1.5 μm spacer to form a BiNem®-type cell, and after adding chiral dopant S-811 (Merck) to achieve a spontaneous pitch of 6 μm at 25° C. in the liquid crystal composition (H) shown below, it is injected into the cell.

A plurality of measurement temperatures are set in order to measure the operating temperature, and at each temperature a voltage with a pulse width of 5 ms is applied to the cell at 1 V steps until a maximum 30 V. By measuring the anchoring breaking voltage, the actuation voltage is set and the operating temperature range is evaluated.

The property values of this liquid crystal composition (H) are (T_(N-I))=80.5° C., (T_(X-N))=−30° C., and nematic liquid crystal temperature range (ΔT_(N))=110.5° C. Also, the dielectric anisotropy (∈₀Δ∈)=2.35×10⁻¹⁰ F/m, and the refractive index anisotropy (Δn)=0.123. Accordingly, the product of Δn and d of this cell is 184.5 nm.

The operating temperature range of the liquid crystal composition (H) in this BiNem®-type cell is 0 to 66° C., and the operating temperature width is 66° C. The threshold voltage U_(Z) was 18 V at 20° C. This threshold voltage U_(Z), converted to U_(λ/4) when the product of Δn and d is 140 nm, is 13.7 V. U_(λ/4) when the product of Δn and d is 140 nm is converted by the following equation.

$\begin{matrix} {\left( {{Eq}.\mspace{14mu} 4} \right)\mspace{675mu}} & \; \\ {U_{\lambda/4} = {U_{z} \times \frac{140\mspace{14mu}{nm}}{\Delta\;{n \cdot d}}}} & (4) \end{matrix}$

Example 2

The liquid crystal composition (H) in Example 1 was converted to liquid crystal composition (I) shown below and subjected to a similar evaluation.

The property values of this liquid crystal composition (I) are (T_(N-I))=78.0° C., (T_(X-N))=−28° C., and nematic liquid crystal temperature range (ΔT_(N))=106.0° C. Also, the dielectric anisotropy (∈₀Δ∈)=1.93×10⁻¹⁰ F/m, and the refractive index anisotropy (Δn)=0.1048. Accordingly, the product of Δn and d of this cell is 157.2 nm.

The operating temperature range of the liquid crystal composition (I) in this BiNem®-type cell is 0 to 65° C., with the operating temperature width being 65° C. The threshold voltage U_(Z) was 18.0 V at 20° C. This threshold voltage U_(Z), converted to U_(λ/4) when the product of Δn and d is 140 nm, is 16.0 V.

Example 3

The liquid crystal composition (H) in Example 1 was converted to liquid crystal composition (J) shown below and subjected to a similar evaluation.

The property values of this liquid crystal composition (J) are (T_(N-I))=73.6° C., (T_(X-N))=−25° C., and nematic liquid crystal temperature range (ΔT_(N))=98.6° C. Also, the dielectric anisotropy (∈₀Δ∈)=3.39×10⁻¹⁰ F/m, and the refractive index anisotropy (Δn)=0.1269. Accordingly, the product of Δn and d of this cell is 190.35 nm.

The operating temperature range of the liquid crystal composition (J) in this BiNem®-type cell is −10 to 60° C., with the operating temperature width being 70° C. The threshold voltage U_(Z) was 12.0 V at 20° C. This threshold voltage U_(Z), converted to U_(λ/4) when the product of Δn and d is 140 nm, is 8.8 V.

Example 4

The liquid crystal composition (H) in Example 1 was converted to liquid crystal composition (K) shown below and subjected to a similar evaluation.

The property values of this liquid crystal composition (K) are (T_(N-I))=76.0° C., (T_(X-N))=−34° C., and nematic liquid crystal temperature range (ΔT_(N))=110.0° C. Also, the dielectric anisotropy (∈₀Δ∈)=1.87×10⁻¹⁰ F/m, and the refractive index anisotropy (Δn)=0.1334. Accordingly, the product of Δn and d of this cell is 200.1 nm.

The operating temperature range of the liquid crystal composition (K) in this BiNem®-type cell is −5 to 70° C., with the operating temperature width being 75° C. The threshold voltage U_(Z) was 13.0 V at 20° C. This threshold voltage U_(Z), converted to U_(λ/4) when the product of Δn and d is 140 nm, is 9.1 V.

Example 5

The liquid crystal composition (H) in Example 1 was converted to liquid crystal composition (L) shown below and subjected to a similar evaluation.

The property values of this liquid crystal composition (L) are (T_(N-I))=82.2° C., (T_(X-N))=−15° C., and nematic liquid crystal temperature range (ΔT_(N))=97.2° C. Also, the dielectric anisotropy (∈₀Δ∈)=1.68×10⁻¹⁰ F/m, and the refractive index anisotropy (Δn)=0.108. Accordingly, the product of Δn and d of this cell is 162 nm.

The operating temperature range of the liquid crystal composition (L) in this BiNem®-type cell is 0 to 70° C., with the operating temperature width being 70° C. The threshold voltage U_(Z) was 16.0 V at 20° C. This threshold voltage U_(Z), converted to U_(λ/4) when the product of Δn and d is 140 nm, is 13.8 V.

Example 6

The liquid crystal composition (H) in Example 1 was converted to liquid crystal composition (M) shown below and subjected to a similar evaluation.

The property values of this liquid crystal composition (M) are (T_(N-I))=74.0° C., (T_(X-N))=−40° C., and nematic liquid crystal temperature range (ΔT_(N))=114.0° C. Also, the dielectric anisotropy (∈₀Δ∈)=1.94×10⁻¹⁰ F/m, and the refractive index anisotropy (Δn)=0.1077. Accordingly, the product of Δn and d of this cell is 161.55 nm.

The operating temperature range of the liquid crystal composition (M) in this BiNem®-type cell is −3 to 68° C., with the operating temperature width being 71° C. The threshold voltage U_(Z) was 14.0 V at 20° C. This threshold voltage U_(Z), converted to U_(λ/4) when the product of Δn and d is 140 nm, is 12.1 V.

Example 7

The liquid crystal composition (H) in Example 1 was converted to liquid crystal composition (N) shown below and subjected to a similar evaluation.

The property values of this liquid crystal composition (N) are (T_(N-I))=75.3° C., (T_(X-N))=−30° C., and nematic liquid crystal temperature range (ΔT_(N))=103.5° C. Also, the dielectric anisotropy (∈₀Δ∈)=1.98×10⁻¹⁰ F/m, and the refractive index anisotropy (Δn)=0.109. Accordingly, the product of Δn and d of this cell is 163.5 nm.

The operating temperature range of the liquid crystal composition (N) in this BiNem®-type cell is 0 to 65° C., with the operating temperature width being 65° C. The threshold voltage U_(Z) was 18.5 V at 20° C. This threshold voltage U_(Z), converted to U_(λ/4) when the product of Δn and d is 140 nm, is 15.8 V.

Comparative Example 1

The liquid crystal composition (H) in Example 1 was converted to liquid crystal composition (P) shown below and subjected to a similar evaluation.

The property values of this liquid crystal composition (P) are (T_(N-I))=61.3° C., (T_(X-N))=−25° C., and nematic liquid crystal temperature range (ΔT_(N))=86.3° C. Also, the dielectric anisotropy (∈₀Δ∈)=2.28×10⁻¹⁰ F/m, and the refractive index anisotropy (Δn)=0.157. Accordingly, the product of Δn and d of this cell is 235.5 nm.

The operating temperature range of the liquid crystal composition (P) in this BiNem®-type cell is −0 to 50° C., with the operating temperature width being 50° C. The threshold voltage U_(Z) was 14.1 V at 20° C. This threshold voltage U_(Z), converted to U_(λ/4) when the product of Δn and d is 140 nm, is 8.4 V.

Comparative Example 2

The liquid crystal composition (H) in Example 1 was converted to liquid crystal composition (Q) shown below and subjected to a similar evaluation.

The property values of this liquid crystal composition (Q) are (T_(N-I))=61.0° C., (T_(X-N))=−20° C., and nematic liquid crystal temperature range (ΔT_(N))=81.0° C. Also, the dielectric anisotropy (∈₀Δ∈)=2.57×10⁻¹⁰ F/m, and the refractive index anisotropy (Δn)=0.156. Accordingly, the product of Δn and d of this cell is 234 nm.

The operating temperature range of the liquid crystal composition (Q) in this BiNem®-type cell is 5 to 50° C., with the operating temperature width being 45° C. The threshold voltage U_(Z) was 14.2 V at 20° C. This threshold voltage U_(Z), converted to U_(λ/4) when the product of Δn and d is 140 nm, is 8.5 V.

Comparative Example 3

The liquid crystal composition (H) in Example 1 was converted to liquid crystal composition (R) shown below and subjected to a similar evaluation.

The property values of this liquid crystal composition (R) are (T_(N-I))=88.4° C., (T_(X-N))=−44° C., and nematic liquid crystal temperature range (ΔT_(N))=132.4° C. Also, the dielectric anisotropy (∈₀Δ∈)=8.50×10⁻¹¹ F/m, and the refractive index anisotropy (Δn)=0.107. Accordingly, the product of Δn and d of this cell is 160.5 nm.

The threshold voltage U_(Z) of the liquid crystal composition (R) in this BiNem®-type cell was 72.0 V at 20° C. This threshold voltage U_(Z), converted to U_(λ/4) when the product of Δn and d is 140 nm, is 62.8 V.

Comparative Example 4

The liquid crystal composition (H) in Example 1 was converted to liquid crystal composition (S) shown below and subjected to a similar evaluation.

The property values of this liquid crystal composition (S) are (T_(N-I))=91.6° C., (T_(X-N))=−44° C., and nematic liquid crystal temperature range (ΔT_(N))=135.6° C. Also, the dielectric anisotropy (∈₀Δ∈)=9.21×10⁻¹¹ F/m, and the refractive index anisotropy (Δn)=0.120. Accordingly, the product of Δn and d of this cell is 180 nm.

The threshold voltage U_(Z) of the liquid crystal composition (R) in this BiNem®-type cell was 44.0 V at 20° C. This threshold voltage U_(Z), converted to U_(λ/4) when the product of Δn and d is 140 nm, is 34.2 V.

Table 1 shows the results of evaluating the property values of the liquid crystal compositions prepared in the Examples and Comparative Examples described above, and the results of evaluating the properties of the liquid crystal displays fabricated using these liquid crystal compositions.

TABLE 1 Exam- Exam- Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 4 ple 5 Example 6 ple 7 Example 1 Example 2 Example 3 Example 4 T_(N−1) (° C.) 80.5 78.0 73.6 76.0 82.2 74.0 75.3 61.3 61.0 88.4 91.6 T_(X−N) (° C.) −30 −28 −25 −34 −15 −40 −30 −25 −20 −44 −44 ΔT_(N) (° C.) 110.5 106.0 98.6 110.0 97.2 114.0 103.5 86.3 81.0 132.4 135.6 Δn 0.123 0.1048 0.1269 0.1334 0.108 0.1077 0.109 0.157 0.156 0.107 0.120 d (μm) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Δn · d (nm) 184.5 157.2 190.35 200.1 162 161.55 163.5 235.5 234 160.5 180 Δε 26.54 21.78 38.31 21.17 19 21.91 22.4 25.8 29 9.6 10.4 ε₀Δε (F/m) 2.35 × 1.93 × 3.39 × 1.87 × 1.68 × 1.94 × 1.98 × 2.28 × 10⁻¹⁰ 2.57 × 10⁻¹⁰ 8.50 × 10⁻¹¹ 9.21 × 10⁻¹¹ 10⁻¹⁰ 10⁻¹⁰ 10⁻¹⁰ 10⁻¹⁰ 10⁻¹⁰ 10⁻¹⁰ 10⁻¹⁰ operating 0~66 0~65 −10~60 −5~70 0~70 −3~68 0~65 0~50 5~50 temperature range (° C.) operating 66 65 70 75 70 71 65 50 45 temperature width (° C.) U_(Z) (V) 18.0 18.0 12.0 13.0 16.0 14.0 18.5 14.1 14.2 72.0 44.0 U_(λ/4) (V) 13.7 16.0 8.8 9.1 13.8 12.1 15.8 8.4 8.5 62.8 34.2 at 140 nm

Comparing the Examples and Comparative Examples, it can be seen that the operating temperature range of the BiNem®-type cell is wider for the Examples. That is, with the combination of liquid crystal materials that is the present invention, it is possible to obtain a liquid crystal composition having a wide operating temperature range in a bistable nematic liquid crystal display.

While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.

INDUSTRIAL APPLICABILITY

The liquid crystal composition of the present invention can be suitably utilized for a bistable nematic liquid crystal display in which the composition is confined between two substrates of which at least one of the substrates has a weak zenithal anchoring, and can obtain a bistable nematic liquid crystal display having a wide operating temperature range. 

1. A nematic liquid crystal composition comprising: a) at least 20% by weight of one or more compounds selected from the compound group A represented by general formulas AI to AV and at least 5% by weight of the compound represented by general formula AI, wherein X¹ represents a cyano group, Z¹ represents a single bond, and Y¹ and Y² each represents fluorine atoms:

wherein: R¹ is an alkyl group or alkenyl group having 2 to 7 carbon atoms, in which, in addition, one or more methylene groups existing in the alkyl group or alkenyl group may be substituted each independently with an —O—, —COO— or —OCO— where O atoms are not directly linked to each other, Z¹ represents —CH₂CH₂—, —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, or a single bond, Z² represents —CH₂CH₂—, —CH═CH—, —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, or a single bond, Z³ represents —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, —C≡C—, or a single bond, Z⁴ and Z⁵ are each, independently of one another, —CH₂CH₂—, —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, or a single bond, X¹ is a cyano group, a fluorine atom, a chlorine atom, a trifluoro methyl group, a trifluoro methoxy group, or a difluoromethoxy group (—OCHF₂), Y¹, Y², Y³, and Y⁴ are each, independently of one another, a hydrogen atom, a fluorine atom, a chlorine atom, a trifluoro methyl group, or a trifluoro methoxy group, ring T¹ is a 1,4-phenylene group or a 1,4-cyclohexylene group, in which among these groups the 1,4-phenylene group may be unsubstituted or may have one or more of a fluorine atom, a chlorine atom, a methyl group, a trifluoro methyl group, or a trifluoro methoxy group as a substituent group, m is 0 or 1, and p and q are 0 or 1, provided that p+q is 0 or 1; b) 5 to 50% by weight of one or more compounds selected from the compound group B:

wherein: R² and R³ are each, independently of one another, an alkyl group having 1 to 7 carbon atoms or an alkenyl group having 2 to 7 carbon atoms, in which, in addition, one or more methylene groups existing in the alkyl group or alkenyl group may be substituted each independently with an —O—, —COO—or —OCO— where O atoms are not directly linked to each other, Z⁶ is —CH₂CH₂—, —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, or a single bond, Z⁷ is —CH₂CH₂—, —CH═CH—, —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, or a single bond, Z⁸ is —COO—, —OCO—, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, —C≡C—, or a single bond, Z⁹ and Z¹⁰ are each, independently of one another, —CH₂CH₂—, —COO—, —OCO—, or a single bond Y⁷ to Y¹⁴ are each, independently of one another, a hydrogen atom, a methyl group, a fluorine atom, or a chlorine atom, and rings T² and T³ are each, independently of one another, a 1,4-phenylene group or a 1,4-cyclohexylene group, in which among these groups the 1,4-phenylene group may be unsubstituted or may have one or more of a fluorine atom, a chlorine atom, a methyl group, a trifluoro methyl group, or a trifluoro methoxy group as a substituent group, wherein the relative proportions of the above-mentioned compounds used in the nematic liquid crystal composition is determined to simultaneously obtain the following physical characteristics for the composition: a nematic-isotropic liquid transition temperature (T_(N-I)) of the liquid crystal composition of greater than or equal to 50° C., a nematic temperature range (ΔT_(N)) of the liquid crystal composition of greater than or equal to 50° C., a dielectric anisotropy of larger than or equal to 8×10⁻¹¹ F/m at 20° C., and a weak zenithal anchoring force on at least one of two substrates for a bistable nematic liquid crystal display that confine the composition, being defined by an anchoring breaking voltage U_(λ/4) of less than or equal to 25 volts for a cell with a thickness (d) such that the product (Δn·d) of the refractive index anisotropy (Δn) and the thickness (d) is 140 nm at 20° C.
 2. The nematic liquid crystal composition according to claim 1, wherein the dielectric anisotropy at 20° C. is 1.3×10⁻¹⁰ F/m to 3.0×10⁻¹⁰ F/m.
 3. The nematic liquid crystal composition according to claim 1, characterized by containing two or more compounds selected from the compound group A.
 4. The nematic liquid crystal composition according to claim 1, wherein in general formula AI, X¹ represents a cyano group, Z¹ represents a single bond, and Y¹ and Y², independently of one another, represent a hydrogen atom or a fluorine atom, in general formula AII, X¹ represents a cyano group, a trifluoro methoxy group, or a fluorine atom, Z¹ represents —COO—, —OCO—, —CH₂CH₂—, —CF₂O—, —OCF₂—, or a single bond, and Y¹ and Y², independently of one another, represent a hydrogen atom or a fluorine atom, in general formula AIII, X¹ represents a cyano group or a fluorine atom, Z² represents —CH₂CH₂—, —CH═CH—, or a single bond, and Y¹ and Y², independently of one another, represent a hydrogen atom or a fluorine atom, in general formula AIV, X¹ represents a cyano group or a fluorine atom, Z¹ represents a single bond, Z³ represents —COO—, —OCO—, —CF₂O—, —OCF₂—, or a single bond, and Y¹ and Y², independently of one another, represent a hydrogen atom or a fluorine atom, and in general formula AV, X¹ represents a cyano group or a fluorine atom, Z² and Z⁴ represent single bonds, and Y¹ and Y², independently of one another, represent a hydrogen atom or a fluorine atom.
 5. The nematic liquid crystal composition according to claim 1, wherein the refractive index anisotropy (Δn) at 20° C. is within the range of 0.06 to 0.13.
 6. The nematic liquid crystal composition according to claim 1, wherein the refractive index anisotropy (Δn) at 20° C. is within the range of 0.06 to 0.13, and the dielectric anisotropy is within the range of 1.3×10⁻¹⁰ to 3.0×10⁻¹ to F/m.
 7. The nematic liquid crystal composition according to claim 1 used for a single polarizing plate mode, wherein the refractive index anisotropy (Δn) at 20° C. is within the range of 0.06 to 0.13.
 8. A bistable nematic liquid crystal display comprising: the nematic liquid crystal composition recited in claim 1 being situated between two substrates, forming a cell, and provided with alignment films and electrode structures on the inner sides of said substrates, wherein at least one alignment film has a weak zenithal anchoring defined by an anchoring breaking voltage U_(λ/4) being less than or equal to 25 volts for a cell with a thickness d such that the product of the refractive index anisotropy (Δn) and the thickness (d) is 140 nm (at a temperature of 20° C.), thereby permitting the liquid crystal composition to adopt at least two different stable states and to switch between the two stable states when a suitable electric signal is applied to said electrode structures.
 9. The bistable nematic liquid crystal display according to claim 8, which is of the anchoring breaking type.
 10. The bistable nematic liquid crystal display according to claim 8, wherein at least two of the different stable states include a first stable texture that is uniform or slightly twisted, in which the molecules of the liquid crystal composition are aligned at least approximately parallel within the range of 0±20° to each other, and a second stable texture that is twisted between the two substrates by 180°±20°, and switching between the first stable texture and the second stable texture is achieved when a suitable electric signal is applied to said electrode structures.
 11. The bistable nematic liquid crystal display according to claim 10, wherein the switching between the two stable textures is achieved by breaking the zenithal anchoring on at least one of the substrates.
 12. The bistable nematic liquid crystal display according to claim 8, wherein the anchoring by the alignment films of the two substrates is monostable for both.
 13. The nematic liquid crystal composition according to claim 2 used for a single polarizing plate mode, wherein the refractive index anisotropy (Δn) at 20° C. is within the range of 0.06 to 0.13.
 14. The nematic liquid crystal composition according to claim 3 used for a single polarizing plate mode, wherein the refractive index anisotropy (Δn) at 20° C. is within the range of 0.06 to 0.13.
 15. The nematic liquid crystal composition according to claim 4 used for a single polarizing plate mode, wherein the refractive index anisotropy (Δn) at 20° C. is within the range of 0.06 to 0.13. 